Below these are totally hypothetical numbers and not realistical and I am using them to keep it simple and understandable. If you use an iPhone charger it will show the iPad is charging now look above at the numbers and you see that it is about 1/2 of what the iPad charger is so it will take twice as long to charge up your iPad using an iPhone charger. Reason it says 'Not Charging' when you are looking at the screen when plugged into your PC is the USB is basically powering up your screen and doing nothing for the battery. To test that plug your iPad into your PC USB charger before you go to bed and write down what the battery percentage is and in the morning you will see it went up. Will they all charge an iPad even when your PC shows 'Not Charging'? YES your PC USB will charge your iPad, but very, very slowly because it is 1/4th of what an iPad charger is so it will take 4 times longer than an iPad charger (BUT YOUR SCREEN MUST BE OFF). I can see that a lot of people are confused about chargers due to them showing watts, volts, & amps and since I have a very extensive background in electronics I will clarify all this in this post. I just recently got an iPad and have the iPhone 4, iPhone 4S, iTouch, Ipod, basically I have everything Apple makes.

His tip is reproduced (with permission) below. Merlin1128 recently posted an excellent article about chargers for iPhones and iPads, how they work, and how to use them.

Mac mini power supply output voltage